SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Revenue Recognition) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Unbilled revenues	$ 129,683	$ 86,422	$ 86,500
Deferred tax assets	61,905	24,801	24,974
Liabilities
Accrued expenses and other current liabilities	98,442	89,104	89,812
Stockholders’ equity
Retained earnings	699,568	519,277	518,820
Balance under Revenue Guidance in Effect before Topic 606
Assets
Unbilled revenues			86,500
Deferred tax assets			24,974
Liabilities
Accrued expenses and other current liabilities	98,354		89,812
Stockholders’ equity
Retained earnings	699,637		$ 518,820
Topic 606 | Adjustments
Assets
Unbilled revenues		(78)
Deferred tax assets		(173)
Liabilities
Accrued expenses and other current liabilities	88	(708)
Stockholders’ equity
Retained earnings	$ (69)	$ 457